An aspect of the game where you can get additional content or powerups by interacting with other people who own the game. And then there are the games that can only be played in Multi-Player. Supremely awesome if one actually has access to friends who own the game; a Fan Convention is a great place to do this. Modern consoles and handhelds are able to go online, so many games with these features also include anonymous internet matchmaking. Compare One Game for the Price of Two, where some games require this for Hundred-Percent Completion. Not to be confused with that kind of socialization bonus.